The fracture surfaces are smooth, as is typical of corrosion dissolution. On the other hand, (b) and (d) were obtained at f where crack growth was suppressed. The coarse appearance was due to excessive corrosion dissolution.
Concerning the extent of adhesion of surface corrosion products, there was no appreciable difference between growing cracks and arrested cracks. Thus, it seemed unjustifiable to explain the distinction between crack growth and arrest in terms of the wedge effect due to accumulated corrosion products in the crack.
4. Conclusions
We investigated the surface crack growth rate for mild, and hard steel wires in natural seawater under a variety of loading frequencies (f), and seawater temperature (T) with the aim of evaluating the effect of steel strength on cracking behavior.
At a given T, the environmental acceleration factor for crack growth indicated a peak value at a certain f depending T for both specimens. The maximum acceleration factor tended to increase with decreasing T, and with greater steel strength in order of SWRM10 and SWRH42A. When f is further reduced past the f level yielding the maximum environmental acceleration factor, crack growth tended to slow down and eventually to cease. The threshold f level leading to crack arrest also depended on T and strength of each tested steel : 0.6 Hz for SWRM10 and 0.2 Hz for SWRH42A at 308K, 0.3Hz for SWRM10 and 0.1 Hz for SWRH42A at 303k, and 0.15 Hz for SWRM10 at 298K, exhibiting a decreasing trend with decreasing T and increasing strength of each tested steel.
Judging from these observation results, crack growth behavior for both steels were conducted to depend on the balance between crack growth acceleration due to corrosion dissolution at the crack tip and crack growth deceleration due to rounding of the crack tip. Thus, increased T and excessive decreases in f would promote crack tip rounding, leading to eventual crack arrest through the acceleration of crack wall dissolution with reference to crack tip dissolution. However, suppression of crack growth becomes more difficult as steel strength increases because the crack growth rate is accelerated as steel strength increases.
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